Silicones can be divided according to their areas of application into oils, resins, and rubbers. Silicone oils, which are processed into emulsions, anti-foaming agents, pastes, greases, and the like, are linear polydimethyl siloxanes. Silicone resins are more or less cross-linked polymethyl or polymethyiphenyl siloxanes, whose elasticity and thermal resistance increases with the content of phenyl groups. Silicone resins are processed, for example, to produce lacquers, to coat household utensils, and as laminates. Silicone rubbers are masses that are convertible to the elastic, rubber state, which contain, as basic polymers, polydiorganosiloxanes, which exhibit groups that are accessible to cross-linking reactions. Silicone rubbers differ from other types of rubbers by virtue of the fact that they are not purely organic compounds. Their structure imparts their unique properties to the silicone rubbers. Generally, a distinction is drawn between hot-vulcanizing (HTV) and cold-vulcanizing (RTV) silicone rubbers.
Among the cold-hardening RTV silicone rubber masses, a distinction may be drawn between single and dual component systems. In the case of single component silicone rubber (RTV-1), the mass polymerizes slowly at room temperature under the influence of the moisture in the air, whereby the cross-linking occurs as a result of the condensation of SiOH groups, forming Si—O bonds. The SiOH groups are formed as a result of the hydrolysis of SiX groups of a species that occurs intermedially from a polymer with OH groups in terminal position and a cross-linking agent. In the case of the dual component rubbers (RTV-2), mixtures of silicic acid esters and organic stannous compounds are used.
The HTV silicone rubbers represent, in most cases, materials that can be molded plastically, which contain highly dispersed silicic acid plus organic peroxides as cross-linking catalysts, and yield heat-resistant elastic silicone elastomers (silicone rubber) after vulcanization at temperatures in excess of 100° C. Another cross-linking mechanism consists of the addition of Si—H-groups to Si-bonded vinyl groups, both of which are built into the polymer groups or at their end, respectively. In the case of HTV silicone rubbers, in addition, radiation cross-linking is known. Since 1980, a liquid silicone rubber (LSR) technology has been established, in which two liquid silicone rubber components are vulcanized in injection molding machines by way of addition cross-linking.
Liquid silicone rubbers (LSR's), because of their particular material properties, open up new areas of application in elastomer processing. Thus, new elastomer-thermoplast composites can expand the spectrum of silicone rubbers known thus far.
The technology for large-scale serial production of solid molded bodies from silicone rubber by using liquid silicone is known as injection molding (liquid injection molding, LIM). The liquid injection molding process affords the advantage that complex pieces can be configured in a flexible manner, that pre-heating is not necessary, that after setting, no shrinkage of the molded bodies occurs, and less material is needed.
On injection molding machines (that work completely automatically), first, two liquid-paste-like silicone pre-polymers are thoroughly mixed with one another at room temperature using a static mixer, and then pressed into a mold under high pressure. As a result of heating the mold, because of the addition reaction between the two components, a three-dimensional cross-linking of the silicone pre-polymers to the solid, fully-molded silicone polymer, to the molded body made of silicone, results (see the corporate prospectuses of the firm of Wacker Burghausen regarding ELASTOSIL® LR, or GE BAYER Silicones, Leverkusen, regarding SILOPREN® LSR, or Battenfeld Meinerzhagen regarding injection molding machinery for liquid silicone). Frequently, these injection-molding machines exhibit dosage apparatuses, so-called multi-component dosage apparatuses, for the homogeneous admixture of colors or other additives, to the two liquid-paste-like silicone pre-polymers.
Molded foam bodies made of silicone, silicone rubber, for example, such as profiles or stoppers for bacterial culture bottles, and processes for their production, are known.
The Japanese patent application Sho 44-461 (461/1969) describes moldable, sponge-like silicone rubber compositions that contain a thermally degradable swelling agent, especially azobisisobutyonitrile. As a result of the degradation of the swelling agent, substances are produced that are harmful to human beings and must be regarded, therefore, as problematical from the standpoint of environmental pollution.
The Japanese patent application Hei 10-36544 (36,544/1998) describes a moldable, sponge-like silicone rubber composition that comprises hollow thermoplastic silicone resin particles that are mixed into the liquid silicone composition whereby during the polymerization, gases develop and in this way, produce pores in the molded body. The silicone rubber sponge that is produced in this manner possesses just slight mechanical strength; accordingly, this sponge's uses are limited.
The U.S. Pat. No. 6,299,952 describes a moldable silicone rubber sponge composition, which also comprises thermoplastic resin spherules that contain gas.
Likewise, a process is known in which, initially, liquids having a low boiling point are mixed with silicone pre-polymers that are not cross-linked. At issue in the case of these liquids, for example, are methylene chloride, HALON® (polytetrafluoroethylene), hepatane, and trichloroethylene. The mixture obtained is then filled into a mold in such a manner that the quantity filled makes up just a percentage of the mold's volume. The mold is then exposed to elevated temperatures. In the process, the liquid evaporates, as a result of which the silicone polymer swells up and fills the mold completely. As a result of the temperature increase, the silicone pre-polymer cross-links increasingly to a silicone polymer, so that a molded foam body that has the contours of the mold results. Thus far, this technology has not prevailed for large-scale serial production of molded foam bodies, since the admixture of liquids having a low boiling point to silicone pre-polymers is disadvantageous. The liquids that can be mixed with silicone pre-polymers are either readily flammable, so that work must proceed under conditions that afford protection from explosions, or they are harmful to the environment, such as the chlorinated or fluorinated chain hydrocarbons, FCHC's.